The present invention relates to a polyethylene-2,6-naphthalate film for capacitor. More particularly, it relates to a polyethylene-2,6-naphthalate film for capacitor having excellent heat resistance, moisture resistance and electrical properties and capable of use in the automobile engine room.
Film capacitors are generally manufactured by a method in which a base film such as a biaxially oriented polyethylene terephthalate film or a biaxially oriented polypropylene film and a thin film of a metal such as aluminum are placed one over another and rolled up, a method in which a deposited film of a metal such as aluminum or zinc is formed by vapor metallization on the surface of the said base film and then rolled up or laminated, or method in which a rolled up element is crushed to form a flat-shaped element. Recently, with the request for miniaturization and reducing the thickness of the electrical or electronic circuits, efforts for miniaturization or packaging (mounting) of the capacitors have been made, too. In their applications to the automobiles, the use environment of the capacitors is not limited to the driving cab but is expanded to a high temperature and high humidity situation as in the engine room.
It has been required that a dielectric material used for capacitors can show a large dielectric constant, a less change in properties depending upon temperature or frequency, a high insulation resistance and a good stability even when exposed to various environmental conditions. However, many of the conventional dielectric materials exhibit a moisture absorption property and, therefore, tends to be frequently deteriorated due to the moisture absorbed.
In the conventional capacitors, it is necessary to enclose or package the capacitor in an appropriate outer sheath or casing such as metal casings for preventing any moisture from entering thereinto. Subsequently, when resins capable of showing a high moisture resistance have been developed, resin-dipped type capacitors have been prevalently used instead of the packaged-type ones.
In association with a recent tendency for miniaturization and surface-mounting of capacitors, it has been required to reduce the thickness thereof. Upon reducing the thickness of the capacitors, it is necessary to assure good properties thereof even when the capacitors are simply packaged without dipping in resin. Further, it has been required to provide films for the capacitors which not only show an excellent durability to various environmental conditions upon use such as ambient temperature, humidity or vibration, but also have stable electric characteristics.
For example, electric control units for automobiles have been conventionally disposed at as large a distance as possible from engine to avoid adverse influences thereon. However, due to recent tendency of weight-reduction and high efficiency of automobiles, these units tend to be arranged in close proximity to the engine. As a result, it has also been required that films used in these units are excellent in heat resistance, moisture resistance and durability to vibration or the like. Since polyethylene-2,6-naphthalate films have a higher glass transition temperature and a higher Young's modulus than those of ordinarily used polyethylene terephthalate films, it is expected that when such polyethylene-2,6-naphthalate films are used in capacitors, the temperature range in which the capacitors are usable, can be broadened up to an elevated temperature. However, in specific applications such as upon mounting onto automobiles or the like, in order to obtain capacitors having an excellent durability to severe environmental conditions upon use and a high reliability, it has been strongly demanded to provide films therefor which can show stable dielectric properties, i.e., are excellent in dielectric constant and dielectric loss to impart a high dielectric breakdown voltage thereto. Further, such films used for surface-mounting capacitors have also been required to withstand a high temperature, a high humidity, vibration or the like. Especially, it is necessary that the surface-mounting capacitors exhibit a high moisture resistance and a high heat resistance without any outer sheath in order to achieve miniaturization and small thickness thereof.
For the solution of the problem of heat resistance, a method has been proposed in which intrinsic viscosity and refractive index of the film are properly controlled, but by this method, the above request has not been satisfied.
As a result of the present inventors' earnest studies to solve the above problem, it has been found that a film capacitor made by using a polyethylene-2,6-naphthalate film having the specific properties is suitable for use in the automobile engine room.
The present invention has been attained on the basis of the above finding.